The present invention generally relates to orthopedic implants used for correction of spinal injuries or deformities, and more specifically, but not exclusively, concerns apparatuses for fixing a portion of the spine, such as the cervical spine, to allow correction or healing thereof.
In the field of spinal surgery, it is known to place implants into vertebrae for a number of reasons, including (a) correcting an abnormal curvature of the spine, including a scoliotic curvature, (b) to maintain appropriate spacing and provide support to broken or otherwise injured vertebrae, and (c) perform other therapies on the spinal column.
Typical implant systems utilize a rod as the support and stabilizing member. In such an implant, a series of two or more screws are inserted into two or more vertebrae to be instrumented. A rod is then placed within or coupled to the heads of the screws, or is placed within a connecting device that links the rod and a screw head, and the connections are secured. In this way, a supporting structure is fixed to the vertebrae.
Many varieties of bone fixation screws are mono-axial in construction. That is, such devices are connected to the rod or plate such that a longitudinal axis through the rod or plate and a longitudinal axis through the fixation device are capable of only a single position with respect to each other. While useful in certain circumstances, in many therapeutic situations the degree of precision required to use such an inflexible device is impractical.
More recently, bone fixation devices having multi-axial capability have been introduced. Examples of such constructs are shown in U.S. Pat. Nos. 5,797,911, 5,954,725, 5,810,818 and 6,485,491 which is hereby incorporated by reference. These devices help to reduce the required precision of placement of the fixation device, since the saddle portion of the fixation device is multi-axially positionable on the anchor member portion. The saddle portion can thus be positioned so as to easily receive the rod, limiting or removing much of the positioning difficulty inherent in prior devices.
Most such devices are designed for spinal fixation at the thoracic and lumbar levels and allow only a limited angulation of the anchor member in relation to the saddle member. There is a need in the art for a multi-axial bone attachment assembly, and particularly one that is useful in the cervical region of the spine with a greater degree of such angulation.